The stones are (i believe) a variety of Sarsen, certainly from the evidence at the Valley of the Stones, they seem to have been formed in the same way.
That would make them silcretes, these are unusual i that they are formed fairly close to the surface where the loose grains are cemented together by silica in the groundwater. They are generally formed in tropical conditions where there is a lot of movement up and down.
This process explains the presence of root holes in the Avebury stones and the silicified wood clearly visible in a couple of stones of Avebury's D-feature.
With the Dorset sarsen it would appear there were a lot of flint nodules in the loose chalk soil which was subsequently bound by the silica.
Incidentally this also explains why, although many of the sarsen stones at Avebury have clear solution features on some surfaces they do not appear to have suffered any further solution since there erection, i.e. the solution features were actually formed when the rock was not fully formed.
I think all that's right anyway...